Sequential differential mobility analyzer and method of using same

ABSTRACT

The invention is essentially a sequential (“DMA”) apparatus using a novel arrangement of at least three electrodes and at least two block electrodes to produce a DMA apparatus having at least two sequential DMA regions between pairs of adjacent electrode walls within the same housing. This apparatus is used to improve the transfer of particles into the subsequent DMA region without a vacuum or pump, and to improve the separation of target particles from non-target particles and concentration and collection of the target particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 13/488,174 file on Jun. 4, 2012 which claims thebenefit of U.S. Provisional Application No. 61/493,212, filed Jun. 3,2011, the content of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

RESERVATION OF RIGHTS

A portion of the disclosure of this patent document contains materialwhich is subject to intellectual property rights such as but not limitedto copyright, trademark, and/or trade dress protection. The owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure as it appears in the Patent and TrademarkOffice patent files or records but otherwise reserves all rightswhatsoever.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to a sequential differentialmobility analyzer for separating and concentrating the size of selectedtarget ions or charged target particles (collectively “TargetParticles”). More particularly, the invention disclosed herein primarilyuses a combination of differential aerodynamic mobility and differentiallateral electrical mobility, within sequential regions having bothairflow(s) and electrical field(s), to separate Target Particles (havinga targeted size and electronic charge) from other particles.

A differential mobility analyzer (“DMA”) is an instrument typically usedto separate small charged aerosol particles based on their electricalmobility, for detection and classification. Many DMAs include twocharged concentric cylindrical electrodes, creating an electric fieldbetween adjacent electrode walls. This essentially annular pathway (orannular region) between adjacent electrodes may be considered theanalysis region. Also included is an aerosol inlet for introducingsample particles (including Target Particles) into the instrument. Asheath gas inlet permits sheath gas (or sheath gas, collectively “sheathgas”) to flow into the instrument between the electrodes, which drawsthe polydispersed particles through the annular region.

In most cases, the resolution of the DMA is limited by diffusion,turbulence, initial spatial distribution of particles, and the ratio ofaerosol flow to sheath flow which relates to the transfer function ofthe particles.

One disadvantage with using only two electrodes is that traditionallythere is only one drift region of electrical field inducing differentiallateral drift of different particles due to each particle's electricalcharge and aerodynamic diameter.

Another disadvantage with using traditional tandem or sequential DMAs isthat they do not include a plurality of analysis regions within the samehousing, and fewer target particles will be separated en route to theinstrument exit. The typical sequential DMA setup will not materiallyincrease the resolution above that of its individual DMA components.

Yet another disadvantage of a regular DMA is that sheath flow throughthe gap in electrodes is directed inwardly toward the central exit. Thisis required to improve the particle transport efficiencies only becauseclassified aerosol flow which contains only target particles aresuctioned by the external pump. This flow direction reduces theresolving power of the instrument because it does not prevent thediffusive crossing of unwanted particles (including neutral particles).

Another disadvantage of existing DMAs is that target particles arediluted because the classified aerosol flow rate is high or fast, toachieve sufficient transport efficiencies. Consequently, possiblecoupling devices such as a mass spectrometer cannot utilize all theparticles in the classified aerosol flow. In this regard, the detectionefficiencies are severely limited.

Background of the Invention

The electrical mobility of a charged particle is inversely related tothe particle's size; smaller particles exhibit greater mobility withinan electrical field than do larger particles (of like charge).Conversely, larger particles travel more in a “downwind” directionduring its longer residence time in the drift region due to theirsmaller electrical mobilities. By calibrating and coordinating theparameters of both the airflow and the gradient(s) of the electricalfield(s) transversing the airflow route(s), smaller-than-targetedparticles can be electronically attracted while larger-than-targetedparticles continue being swept downstream with the airflow, so that onlythe Target Particles exit the instrument. Ideally only the TargetParticles, having the desired electrical mobility and particle size, areextracted from the analyzer.

The following patents are arguably material to the patentability of theinvention disclosed herein:

patent/ Date of Issue/ application No. 1^(st) Inventor Publication6,607,597 Sun et al. Aug. 19, 2003 6,787,763 De La Mora et al. Sep. 7,2004 7,161,143 De La Mora et al. Jan. 9, 2007 7,213,476 Cheng et al. May8, 2007 7,521,673 Areas et al. Apr. 21, 2009 7,723,677 Ramiro Areas etal. May 25, 2010

U.S. Pat. No. 7,723,677 issued to Ramiro Arcas et al. essentiallydiscloses a DMA having an electric field component opposite to the dragflow to cause the main electric field to be oblique to the velocityfield of the drag flow, rather than perpendicular to the velocity fieldof the drag flow. It discloses a control volume with a rectangular basein which two opposing walls made up of electrodes define an electricfield. The two remaining opposite sides of the region form an inlet andoutlet of the ordinary cross flow, which is perpendicular to theelectrodes. It also discloses the usage of resistive electrodes orconductive electrodes separated by insulators to achieve an electricfield against the sheath flow inside the controlled volume. With theexternal circuit being open or closed, the controlled volume can beswitched from classic DMA to DMA utilizing oblique fields against thesheath flow. The device contains shared controlled volume as well as asingle inlet with multiple exit slits. One of the exit slits locatedupstream is used when the device is used as DMA with oblique field.

U.S. Pat. No. 7,213,476 issued to Cheng et al. essentially discloses amulti-stage DMA for aerosol measurements including a first electrodehaving at least one inlet for receiving an aerosol including chargedparticles for analysis. A second electrode is spaced apart from thefirst electrode, and has at least one sampling outlet disposed at aplurality of different distances along its length. A volume between thefirst and second electrode between the inlet and one of the outletsforms a classifying region, with the first and second electrodes forcharging to suitable potentials to create an electric field within theclassifying region. The inlet in the first electrode receives a sheathgas flow at an upstream end of the classifying region, wherein eachsampling outlet functions as an independent DMA stage and simultaneouslyclassifies different size ranges of charged particles based on electricmobility. The aerosol is preferably injected from a central electrodeand the sampling flow is preferably withdrawn through an outerelectrode.

None of the cited patents expressly disclose a sequential DMA analyzerhaving a housing enclosing electrodes forming a plurality of sequentialDMA analysis regions without overlap of controlled volume for analyzinga Target Particle, with the sample aerosol intended to initially traveldownstream with the sheath flow without pump assistance, and including aplurality of guide electrodes for guiding Target Particles to the exitoutlet.

SUMMARY OF THE INVENTION

Although the present invention has several embodiments, the versiongenerally described is essentially a method and apparatus for separatingcharged Target Particles or ions in a sequential differential mobilityanalyzer. The apparatus essentially comprises (includes) a housingenclosing a novel arrangement of electrodes forming a 1st DMA analysisregion and a 2^(nd) DMA analysis region, and utilizing guide electrodesfor guiding Target Particles between the DMA regions and toward an exitwhile diverting non-target particles. Sheath gas is applied through asheath gas inlet to facilitate particle movement in an essentiallylinear downstream direction, preferably via laminar airflow. The gas orair sample, containing both Target Particles and non-target particles,is introduced into the apparatus through an upstream sample inlet. Thetwo DMA analysis regions are formed between pairs of adjacent electrodewalls within the same housing. This apparatus is used to improve thetransfer of particles into the subsequent DMA analysis region withminimal volume flow rate of carrier gas containing highly concentratedpolydispersed aerosol, to improve the separation of Target Particlesfrom non-target particles, and to otherwise improve the DMA resolutionand analytic capabilities.

An electric field is established in each analysis region between theelectrodes, by DC voltage power supply. Electrical mobility is theability of charged particles to travel through a medium in response toan electrical field that is attracting or repelling them.

Using the labels of FIG. 4, as the particles travel along theessentially annular pathway between adjacent pairs of electrodes(grounded-housing 0 and medial-electrode 1), non-target particles eithermigrate downstream or are lost at the electrode wall (1,1) and (1,2),leaving particles having an electrical mobility closer in range to theTarget Particles' electrical mobility. A gap exists between the upstreamsegment of the medial electrode (1,1) and the downstream segment of themedial electrode (1,2) to allow these particles to continue travellingtoward the exit for capture or analysis. At this junction, anotherelimination step occurs and the smaller particles migrate toward theupper segment of the central electrode (2,1), some perhaps migratingupstream toward the downstream tip of the upper segment of the centralelectrode (2,1). The remaining particles travel into the 2nd DMAanalysis region, where non-target particles are primarily attracted tothe middle section of the central electrode (2,2) and the lower sectionof the central electrode (2,3). The smaller particles will be furtherattracted to (and eliminated by) electrodes (3,2) and exit electrode(3,3). At the final elimination stage, only the Target Particles areavailable for extraction at the exit outlet.

In general, the invention disclosed herein includes an improveddifferential mobility analyzer apparatus for analyzing a sample ofairborne particles, said apparatus comprising a housing encompassing aplurality of concentric electrodes having walls defining a plurality ofairflow pathways and flow rates and a plurality of electrical fieldstherein for facilitating differential movement of airborne particlesfrom an upstream end of the housing toward a downstream exit endcomprising a central exit electrode-tip. Each respective electrode wallalso includes a gap allowing lateral drifting of some of the airborneparticles from an outer of the airflow pathways into an inner of saidairflow pathways enroute to the exit electrode-tip. The upstream end ofthe housing further comprises a sample gas inlet providing the sample ofpolydispersed aerosol particles to an outermost first of said airflowpathways, and a sheath gas inlet providing sheath gas to all of theairflow pathways.

More particularly, the housing includes a cylinder having a groundedhousing sidewall; the plurality of electrodes includes a concentriccylindrical arrangement within the grounded housing sidewall. One of theelectrodes includes a medial electrode nearest the housing having anupstream segment and a downstream segment separated by a midstream gap.The downstream segment terminates in the end wall. The medial electrodeand the grounded housing sidewall define the first airflow pathwaytherebetween.

One of the electrodes may be a central electrode (2) within the medialelectrode, and having an upper section and a middle section separated bya middle gaplet, such as the first outer opening between the first outerwall and the second outer wall. The upper section of the centralelectrode includes a gaplet-tip, an extension of the first inner wall,that extends further downstream than the upstream segment of the medialelectrode. One primary purpose of the gaplet-tip is to attract andassist elimination of non-target particles that migrate through the gap,even pulling some of such particles upstream against the sheath flowunder the appropriate combination of voltage, sheath flow and particlesize. In addition, it reduces the Target Particle loss at downstreammedial electrodes by pulling the Target Particles inward and away fromthe downstream segment of medial electrode. The upper section of thecentral electrode also includes an electrical voltage substantially morenegative than that of the upstream segment of the medial electrode. Theaggregate downstream length of the gaplet-tip and the gaplet isapproximately that of the midstream gap between the upper and lowersegments of the medial electrode. In one embodiment, the gaplet-tipextends approximately four-tenths of the downstream length of the gap.

The downstream segment of the medial electrode and the middle section ofthe central electrode and the first block electrode (3,1) commence atessentially the same relative position immediately downstream of thegap. The potential applied to middle section of the central electrodemay be set at more negative than that of the opposite portion(s) of thedownstream segment of the medial electrode but less than that of thesecond block electrode (3,2). The central electrode also has a lowersection separated from the middle section by a cleft, with the lowersection of the central electrode terminating downstream in the end wall.The central electrode and the medial electrode define a second airflowpathway therebetween. The central electrode defines a third airflowpathway to the exit electrode-tip; a first block electrode (3,1) and asecond block electrode (3,2) are situated within the third airflowpathway. The apparatus preferably includes a means of selecting thevoltage applied separately to each of the segments and sections andportions.

One primary object of the present invention is to provide at least twonon-overlapping sequential DMA analysis regions within the same housingto improve the transfer of particles from one DMA region into a secondDMA region without a vacuum or with the minimal assist of a vacuum, thusmaking this apparatus ideal for a mass spectrometer inlet.

Another primary object of the present invention is to provide at leasttwo sequential DMA regions within the same housing to improve separationof Target Particles from non-target particles.

Another object of the invention is to provide a DMA apparatus that doesnot require the use of a pump for classified sample aerosol flow.

Another object of the invention is to provide a guide electrode toenhance the highly efficient transfer of particles into the second innerDMA.

Another object of the present invention is to separate particles usingelectrical fields which induce particle movement independent of thatcreated by the sheath flow.

Other objects will be apparent from a reading of the written descriptiondisclosed herein, together with the claims. It would be advantageous touse an analyzer having at least three electrodes, two block electrodes,and at least two DMA regions within the same housing for detection,classification and concentration of Target Particles to maximize bothtransfer and isolation efficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the airflow of two operational modes, with the left-sideshowing an outward cross flow mode and the right-side showing a no crossflow mode.

FIG. 2 shows the separation of groups of ions having different size, bythe version of the invention depicted in FIG. 4.

FIG. 3 shows the separation of the groups of ions having same m/z ratio,by the version of the invention depicted in FIG. 4.

FIG. 4 depicts a longitudinal cross-section view of anotherrepresentative sample of the apparatus, with some electrode segments orsections (such as the downstream segment of electrode 2) divided intoportions (such as [1,2] and [1,3] and [1,4]).

FIG. 5 depicts a cross-section view, with particles paths, by theversion of the invention depicted in FIG. 7.

FIG. 6 depicts a 3-dimensional rendering using a cross-flow mode DMAapparatus having 19 electrodes with applied voltage, the version of theinvention depicted in FIG. 7. The different colors or shades representsingly charged particles of different mass, shown as 300 kd 325 kD, 400kD, 450 kD, and 500 kD, respectively.

FIG. 7 depicts a longitudinal cross-section view of anotherrepresentative sample of the apparatus, with some electrode segments orsections (such as the downstream segment of electrode 2) divided intoportions (such as [1,2], [1,3], [1,4], [1,5], [1,6], [1,7] and [1,8]).

FIG. 8 depicts a longitudinal cross-section view of one representativesample of the apparatus, illustrating the proportions of downstreamdistances 4:6 of the gaplet-tip and the gap, showing the longitudinallengths of the first outer aperture compared to the first inner apertureand the extension of the second block electrode downstream of the mostlongitudinal upstream position of the second inner aperture. FIG. 8 alsoshows the longitudinal distances 4:6 between the most downstreamposition of the first inner aperture and first outer aperture comparedto the second inner aperture.

FIG. 9 depicts a 3-dimensional rendering of electrical field gradientsof the apparatus of FIG. 8 having one arrangement of voltages applied torespective electrodes.

FIG. 10 depicts another embodiment of the apparatus as a cross-sectionview, with particles paths and with arrows indicating the approximatelocations of sheath gas inlets and outlets.

FIG. 11 depicts another close-up view of FIG. 10.

FIG. 12 is a schematic primarily illustrating the electrode portionsattracting particles smaller than Target Particles.

FIG. 13 is a schematic of removal of larger particles.

FIG. 14 depicts a perspective longitudinal cross-section view of a 3-Drendering of the invention, showing the sheath gas for the innercylinder, the aerosol entrance (360° slit), a laminator to make laminarstraight air flow, and sheath gas for the outer cylinder.

FIG. 15 depicts a perspective longitudinal cross-section view of a 3-Drendering of FIG. 14.

FIG. 16 depicts a close-up view of a part of FIG. 4, illustrating thatelectrode portion 1,2 extends undivided from the midstream gapdownstream to the downstream wall; a space or insulation materialseparates electrode portion 1,2 from electrode portions 1,3 and 1,4 onthe opposite side, and a space or insulation material separateselectrode portion 1.3 from 1.4. Accordingly, each electrode portion iscapable of having a different voltage/charge than another portion.

These drawings illustrate certain details of certain embodiments.However, the invention disclosed herein is not limited to only theembodiments so illustrated. The invention disclosed herein may haveequally effective or legally equivalent embodiments.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of simplicity and to give the claims of this patentapplication the broadest interpretation and construction possible, thefollowing definitions will apply:

The term “airflow” essentially means the flow of gas which may includeaerosol mixture; unless the context dictates otherwise, any reference to“air” is not limited to the gaseous mixture comprising atmospheric air.

The phrase “differential mobility analyzer” essentially means anapparatus isolating like target particles from an initiallypolydispersed sample using a combination of electrical fields and a flowof carrier gas, and possibly sheath gas.

The term “electrode” essentially means an electrically conductivestructure, or alignment of electrically conductive sections or segments(and/or portions thereof) that together comprise the structure,regardless of whether any section or segment (and/or portion thereof)receives the same electrical voltage as any other. Even though a sectionor segment (and/or portion thereof) may receive an electrical voltagedifferent and/or independent from that of another section or segment(and/or portion thereof), all within the same structural alignment maybe referred to collectively as an electrode; alternatively, anyindividual section or segment (and/or portion thereof) may be referredto separately as an electrode.

The invention disclosed herein is not limited by construction materialsto the extent that such materials satisfy the structural and/orfunctional requirements. For example, any electrode material may be usedso long as it satisfies the function for which it is being used, such asconducting electricity and facilitating formation of an electrical fieldgradient.

Similarly, the invention is not limited to any particular embodimentdescribed or depicted herein. For example, for illustration purposes,this disclosure primarily focuses upon positively charged TargetParticles that are attracted to negatively charged regions of anelectrical field, hence the ranges of negative electrical voltagesdisclosed for such attraction of positively charged Target Particles.However, it should be understood that the invention disclosed hereinalso includes electrical fields necessary for negatively charged TargetParticles.

Mass spectrometry currently allows analyses of particle size up to 100kD safely, owing to the technological advances in ionization methods andinlets. However, mass analysis of large biomolecules and aerosols from amixture samples is generally difficult, especially when the mixture islow in concentration. Efficiencies of transport of such large particlesinto a mass spectrometer in ambient condition are severely hinderedbecause of the limiting flow. Flow field generated by a limiting orificeat ambient condition is weak, and large molecules must be nearly staticto be picked up efficiently by the mass spectrometer. This invention isa mass pre-filter device which produces nearly no flow at the transferpoint. It is also useful to classify mixtures by mass, shape anddensity. The design and its computational results of the invention arealso disclosed herein.

This invention is a high pressure target particle isolator using atleast three concentric electrodes (a housing enveloping a medial innerelectrode which envelops a central inner electrode), each innerelectrode having at least one spacing allowing lateral migration ofTarget Particles from a first outer DMA region to a second inner DMAregion within the same housing. The apparatus does not require using apump for the flow of sample aerosol or sheath gas. Thus, a samplecontaining polydispersed particles is not necessarily pushed or pulledthrough the apparatus by a pump system. A small amount of a pumping,however, may be used for the improvement of the particle transferefficiency. However, this invention improves the transfer of particlesinto the subsequent DMA region without a pump, and improves separationand collection of Target Particles. In typical operation, if any crossflow of sheath gas is desired, such cross flow may be directed from aninner airflow pathway into an outer airflow pathway.

In one embodiment, three concentric tubes or electrodes (a central andmedial tube, and a housing tube) essentially form two sequential DMAregions between pairs of adjacent tube walls, and with an innermostairway exit pathway through the central tube. Airflow from the firstupstream DMA region into the second downstream DMA region is enabled bya gap in the wall of the medial tube.

Two operational modes are shown in FIG. 1, that may vary from atraditional pump assisted inward flow mode. On the left of FIG. 1 is theoutward cross flow mode of the present invention, where some sheath gasflows (or migrates along a pressure gradient) laterally outward throughthe gap in the medial electrode and into the adjoining annular pathwayof an analysis region. On the right side of FIG. 1 is the no cross flowmode, where essentially no sheath gas crosses the gap for entry into theadjoining annular pathway. Streamlines of sheath gas flow, and gasvelocity fields are shown here for two different inlet conditions: a)the inlet condition for the outward cross flow mode creates streamlinescrossing a gap almost perpendicular to the aerosol trajectories and b)the inlet condition for the no cross flow mode generates the flow toprevent streamlines from crossing the gap. In either case, the flow rateat the aerosol exit is kept extremely low while achieving high aerosoltransfer efficiencies. Complete separation of particles havingd_(e)=11.3 nm and 11.7 nm is achieved. The inlet flow profile is assumedgenerated by a “flow laminator”. The pathways of target particles ineach mode are similar except the voltages on electrodes and flow rate ineach of the regions are different. Example voltage and flow rates aregiven in the Table 1.

TABLE 1 Voltage set for no cross flow mode target 500 KD Voltage target500 kD with given flow condition with given flow condition AppliedVoltage Applied Voltage Electrode in Volts (V) Electrode in Volts (V)(0,1) 0 (0,1) 0 (1,1) −297 (1,1) −105 (1,2) −300 (1,2) −105 (1,3) −370(1,3) −820 (1,4) −450 (1,4) −1500 (2,1) −450 (2,1) −1450 (2,2) −380(2,2) −875 (2,3) −500 (2,3) −1800 (3,1) −500 (3,1) −2500 (3,2) −1700(3,2) −4500 (3,3) −4000 (3,3) −6750

FIGS. 8 and 9 show the applied voltages at electrodes (cylindricalwalls) and gap alignments. The pair of inner block electrodes ([3,1] and[3,2]), having voltages of −200 V and −570 V in this embodiment, mayhave solid or mesh construction; and both can be placed at the locationsas in the diagram by any means appropriate. The width of the exitpathway is narrower than the block electrodes.

The first, outermost DMA region includes an outermost electrode andmedial electrode, and serves as a first filtering stage. The outermostDMA region is located between the inner wall of the housing (electrode(0,1) and the outer wall of the medial electrode (1,1) and (1,2); (FIG.4). Each electrode is essentially a concentric tube having anelectrically conductive wall for attracting particles of opposite chargeor repelling particles of the same charge. The electrical field in the1^(st) DMA region can be chosen in such a way that target particles of aparticular electric mobility and aerodynamic diameter will betransported near the gap between the medial electrode (1,1) and medialelectrode (1,2). Immediately after the aerosol sample of polydispersedparticles is injected into the outermost airflow pathway, ideally thelarger-than-target particles are essentially swept downstream by thesheath flow; failing to travel through the lateral gap between themedial electrode (1,1) and (1,2), these larger particles ideally areeither attracted (and bound) to the medial electrode wall (1,2), or theyexit the apparatus in the sheath flow (and likely get trapped in anyfilter before any recirculation of sheath gas). Ideally thesmaller-than-target particles are attracted (and bound) to the medialelectrode (1,1). Non-target particles of size similar to the TargetParticles (along with Target Particles) will enter the gap and migrateinto the 2^(nd) DMA region between the medial electrode (1,3), (1,4) andcentral electrode (2,2) and (2,3); see (FIG. 4).

The medial electrode portions (1,3), (1,4) and central electrodesections (2,2) and (2,3), shown in FIG. 4, continue an air flow pathwaynear the exit end of the apparatus. Preferably there is a space orinsulation material between the sidewalls of medial electrode portions(1,3) and (1,4), and the inner sidewall of electrode portion (1,2);preferably there may also be a space or insulative material between thedownstream end of electrode portion(1,3) and the upstream end ofelectrode portion (1-4). Such material may be insulative or dielectric.Electrode portions (1,3) and (1,4) primarily function to removeuntargeted particles which happened to pass through the first DMA regionand the gap; electrode sections (2,2) and (2,3) may assist is suchendeavors. The first block electrode (3,1) primarily functions toattract and eliminate non-target particles in the central airflowpathway. Downstream in the central airflow pathway is a second blockelectrode (3,2). A cleft between the middle and lower sections of thecentral electrode primarily functions to allow the flow of TargetParticles from the 2nd DMA region into the central airflow pathwayenroute to the exit electrode [3,3]. Under the appropriate combinationof voltage, airflow and particle size, the second block electrode mayfacilitate a sharp turn of migrating particles through the cleft.Particles slightly smaller than Target Particles will be attracted toelectrode (3,2) and eliminated, whereas Target Particles will movetoward the final exit electrode. The primary function of the secondblock electrode (3,2) is to attract (and trap) smaller-than-targetparticles that have escaped the upstream electrodes of both DMA regions.

In some embodiments, each segment of the medial electrode (or tube) mayinclude two concentric almost-adjoining sub-segments separated by adielectric (or insulator). Alternatively, each segment or sub-segmentmay be further divided into rings almost-adjoined end-to-end, separatedby a dielectric (or insulator); one example is electrode portions (1,3)and (1,4). The insulating space between them may be about 200 micron.Upstream ring (1,3) eliminates larger non-target particles so that theseparticles adhere to this ring (1,3).

The block electrodes (3,1) and (3,2) enhance the highly efficienttransfer of particles into the 2nd DMA and exit tube by curving theTarget Particles inward by the electric field. The gaplet-tip alsoassists by being positioned slightly upstream of the middle of the gap,to facilitate efficient transfer of the Target Particles into the second(subsequent) DMA region(s) with more favorable initial positiondistributions of the Target Particles and non-targeted particles, aswell as removal of smaller untargeted particles which happen to passthrough the gap (FIGS. 6-7). The second block electrode (3,2) ispositioned upstream of the exit tube, which is essentially in theinnermost annular flow region. The second block electrode includes acleft-tip 202, an extension of the second block electrode, extendingfurther downstream than the middle section of the central electrode. Theexit electrode-tip defines a lumen having a diameter less than thediameter of the block electrode (3,2).

The position of the block electrodes is such that only smaller particlesget attracted toward them, and either get trapped on the electrode or(if a Target Particle) drift closer to the center of the central airflowpathway to align with the exit electrode. Smaller particles may traveltoward a block electrode against the sheath flow in the transition orannular region, resulting in a sharp low-mass cut-off in the sizedistribution. Larger particles are swept downstream by the continuousflow of sheath gas or eliminated by the high-mass filter electrode(1,3). Electrode (1,4) may prevent the loss of Target Particles atelectrode (2,2) as they enter the second DMA region, by attracting themin the opposite direction. In an embodiment optimized for collectingTarget Particles (rather than identifying or analyzing them), electrode(1,3) may serve as repeller plate for guiding electrode (3,2) to enhancethe efficient Target Particle transfer.

The outer diameter of the exit electrode [3,3] is smaller than thediameter of the second block electrode [3,2] to promote betterelimination of the particles larger than the Target Particles to preventthem from exiting through the exit electrode. The outer wall of thenarrow exit serves as a final filtering stage. Because the second blockelectrodes guide most of the particles into the exit tube, therebyreducing the particle loss at the wall surface, the transfer function ofthe apparatus is improved over current technology.

In one embodiment, the upstream segment and downstream segment of themedial electrode may have an electrical voltage of −100 volts. The uppersection of the central electrode may have an electrical voltage of −500volts; the middle section of the central electrode may have anelectrical voltage of −125 volts; and the lower section of the centralelectrode may have an electrical voltage of −125 volts. The first blockelectrode may have an electrical voltage of −200 volts, and the secondblock electrode may have an electrical voltage of −570 volts. The exitelectrode-tip may have an electrical voltage of −500 volts.

In another embodiment, the controlled volume of the differentialmobility classifier can be constructed as shown in FIG. 4 with appliedvoltage listed in Table 1. However, the voltage on each electrode,section, segment or portion thereof may be different than just recited,and each may vary independently of the others in accordance with theneeds of the user and the circumstances of the project at hand.

FIG. 11 is a longitudinal cross-section view of one embodiment of theapparatus. Particles are guided inward by guide electrode (Arrow 18) andblock electrodes (Arrow 13). Arrow 10 of FIG. 11 represents an electrodethat is a blocker to prevent the particles from going downstream. Arrow11 points to the direction of the sheath flow. Arrow 17 points to theoutside electrode wall. Arrow 18 points to a guide electrode. Arrow 19points to the first DMA region, while Arrow 20 points to the second DMAregion. Arrows 12 and 13 point to the smaller particles that areeliminated by the local electric field produced by the block electrode,which is just enough to move at least some of those particles upstream.The heavier particles will get drifted by sheath flow. Arrow 14 showswhere larger particles are eliminated by the upmost tip or outer surfaceof the exit tube electrode because they are too heavy to get into theexit tube (Arrow 15), which has a smaller diameter (high-mass filter).The apparatus will guide the particles well into the exit tube so notmuch of particle loss occurs there.

A sample of charged polydispersed aerosol particles is injected into theoutermost airflow pathway at inlet 16, which is then mixed with sheathgas. During the 1st DMA filtering stage, a portion of smaller non-targetparticles drift laterally to the wall surface of electrodes (1,1) andeliminated within the 1st DMA region while larger particles areeliminated by the surface of the electrode (1,2). The remaining aerosolparticles then migrate downstream with the sheath flow, at the samevelocity as the sheath flow. The radial velocity of a particle due tothe movement in response to the electrical field is determined by theparticle's electrical mobility, Z, defined by v_(e)=ZΔV where v_(e) isthe electrophoretic migration velocity and ΔV is the magnitude of theelectrical field.

Due to the mobility created by voltage difference between the appliedvoltages of the outermost electrode (0,1) and middle (medial) electrodes(1,1) and (1,2), positively charged particles in the target size rangedrift laterally toward the medial electrode with the electrophoreticmigration velocity. Once the flow rate of the sheath flow and voltagedifference between two electrodes are set in the apparatus, only theparticles with certain electrical mobility are transferred efficientlythrough the gap between the upper and lower segments of the medialelectrode, and into the 2nd DMA region. Particles having differentelectrical mobilities, and lighter than those in the target size, willcollide with the wall of electrode (1,1) and will not make it throughthe gap, and they are thereby separated from the main aerosol stream.

Any non-target particles that incidentally remain in the main airflowstream after passing through the gap are immediately attracted to thewall of the central electrode (2,1) and high mass filter electrode(1,3), and consequently eliminated from the main airflow stream. Thewall of the central electrode (2,1) may also include a gaplet-tipextending downstream past the adjacent gap of the medial electrode wall,for attracting more non-targeted particles that have migrated throughthe gap and into the 2nd DMA region.

Smaller particles migrate quickly to electrode (2,1) due to the localstrong electric field overcoming the aerodynamic force acting on theparticles by the sheath flow, while larger particles enter and traversethe 2nd DMA region due to their larger aerodynamic diameter, or areeliminated by the high-mass filter electrode (1,3). Some non-targetparticles remain on the central electrode wall. The larger non-targetedparticles enter the 2nd DMA region at its outer annular region whileTarget Particles enter the 2nd DMA region at its inner annular region.

FIG. 12 is a schematic of elimination of particles smaller than theTarget Particles. Smaller particles either collide with the wall of themedial electrode just above the gap, or get attracted to the electrodeinside the gap and collide with it, and leave the main aerosol streamand follow the sheath flow out into the recirculation path 204.

At the last filtering stage, the larger non-target particles (thatmigrate through the cleft between the middle and lower sections of thecentral electrode) enter the narrow, converging airway and are sweptaway or lost at the outer surface as well as upmost tip of the exit tubewhile non-target particle slightly smaller than the target particle willget attracted by the second block electrode (3,2), allowing only TargetParticles to exit the apparatus through exit tube. FIG. 13 is aschematic of removal of larger particles. The widths of the arrows arerelative to the concentrations. The larger particles collide with theoutside surface (near the top) of the bottom electrodes (1,2) and/or(2,3) and/or (3,3) immediately below the gap or cleft or swept away bythe sheath flow. FIG. 14 is the 3-dimensional rendering of theinvention. Arrows 21, 24, and 25 point to the sheath air inlets for thecentral cylinder, the medial cylinder and the outer cylinder,respectively. Arrow 22 points to the laminator (flow straightener),while Arrow 23 points to the Aerosol entrance (360° slit).

The migration of the aerosol particles through the gap (or the gaplet,or the cleft) is essentially induced by the electrical field, unlikethose driven by the inward air flow created by pumping the sheath gasout through the exit seen in previous DMA apparati. The air flow createdby such a suction apparatus is no longer plug flow near the gap, whichis often used in the modeling process in the computer and ultimatelyresult in errors in the calculated resolution and precision.

In the 2nd DMA region, the refined aerosol undergoes the sameelectrical/airflow filtration process that occurred in the 1st DMAregion. Ideally, Target Particles flow through the 2nd DMA regionwithout colliding (or bonding) with the surface of the central electrode(2,2). The non-target particles that are smaller than the TargetParticles either collide with the surface of the central electrode wallor are attracted to the second guide electrode, which is placed upstreamof the innermost annular flow region of the exit tube.

The electric field produced by the second guide electrode (3,2) attractsall smaller particles inward (and perhaps against the sheath flow),while the remaining particles travel downstream with the sheath gas.Because the sheath gas flow limits the interaction of the strongelectric field generated by the second guide electrode, the amount ofinward drift is a function of the size of the smaller particles. Largerparticles do not drift laterally as much as the smaller particles, andessentially are swept downstream with the airflow. The remainingparticles that are larger than the Target Particles can be eliminated bythe outer wall of the exit tube. The outer wall of this narrow exitserves as another filter stage for the larger particles. Ideally, onlythe Target Particles exit the apparatus.

The first and second block electrodes (3,1) and (3,2) could beconstructed with metallic, conductive mesh structures or materials tocreate a uniform electric field near them, as well as minimumdisturbance to the uniform plug sheath flow inside the hollow centralelectrode. As mentioned previously, a large portion of Target Particleswithin a polydispersed sample travel sufficiently away from the innerwall of the exit electrode, so that the wall loss of the particles byvan de Waal interaction is minimized, thus achieving the highlyefficient transfer of the Target Particles.

Although it is not necessary to use any external pump to removeparticles, a small amount of outward pumping could be used to assist ina better particle transfer. However, the local flow field should only beaffected near the exit electrode, without interfering with the gap atthe first selection stage. Since the usage of a pump is minimal and theaerosol flow can be greatly reduced because of the highly efficienttransfer, it could also help in achieving the creation of highlymonodispersed aerosol.

Because the first and second guide electrodes will guide most of theparticles into the exit tube, thereby reducing the particle loss at thewall surface, the transfer function of the particles in the first DMAregion is given by:

${{\Omega( {Z,Z} }{*)}} = {\max\{ {0,{\min\{ {1,\frac{\frac{Z}{Z^{*}} + \beta - 1}{\beta - {\beta\delta}},\frac{1 + \beta - \frac{Z}{Z^{*}}}{\beta - {\beta\delta}}} \}}} \}}$${\beta = \frac{Q_{a} + Q_{c}}{Q_{Sh} + Q_{e}}},{\delta = \frac{Q_{a} - Q_{c}}{Q_{a} + Q_{c}}}$

where Qa is the volumetric flow rate of the aerosol flow, Qsh is thevolumetric flow rate of the sheath flow, Qc is the volumetric flow rateof the classified sample flow, and Qe is the volumetric flow rate of theexhaust flow. The value of shows the resolving power of the DMA; while δreveals imbalance between the two flows of aerosol. The probability thata particle of mobility Z will be transmitted from the aerosol flow tothe classified aerosol flow when the instrument is set to classifyparticles of mobility Z* is called the transfer function of theclassifier and donates Ω(Z, Z*).

The transfer function Ω is ideal when β is small but allowable range issomewhat limited when collection efficiency is far from ideal. Efficientparticle transfer and separation reduces the use of gas.

The 2nd DMA region will repeat the same procedure for movement andelimination of non-target particles, however, the second block electrode(3,2) is slightly thicker than the exit tube to promote betterelimination of larger particles. The inner radius of the exit tubeelectrode is narrower to promote better separation from largerparticles.

Any of the variables or combinations of the variables, voltage appliedto electrodes, sheath gas in the 1st DMA region, sheath gas in the 2ndDMA region, can be scanned for the analysis of the particle distributionin polydispersed particle with appropriate particle detector.

Typical differential mobility analyzers require a significant amount ofclassified aerosol flow (Q_(c)) to achieve satisfactory transferefficiency. However, low flow rate Q_(c) is desired for massspectrometers, and finding a good balance of efficiency and resolvingpower is difficult in some cases. Any inward cross flow could wronglyguide larger non-target molecules through the gap if they happen todiffuse toward it during the flight.

In another embodiment, the DMA apparatus are constructed as shown inFIG. 7. Here, electrodes (1,3) and (2,2) in FIG. 4 are replaced byplural electrodes shown in FIG. 7. The list of applied voltages islisted in Table 2.

TABLE 2 Applied Voltage in Electrode Volts (V) [0,1] 0 [1,1] −105 [1,2]−110 [1,3] −205 [1,4] −210 [1,5] −215 [1,6] −220 [1,7] −225 [1,8] −300[2,1] −250 [2,2] −255 [2,3] −260 [2,4] −265 [2,5] −270 [2,6] −275 [2,7]−285 [3,1] −500 [3,2] −495 [3,3] −500The primary purpose of having each such section or segment (and/orportion thereof) that can receive an electrical voltage different and/orindependent from that of another section or segment (and/or portionthereof) is to assist a forward motion of particles in relatively slowsheath gas, typically in the same direction as the sheath gas flow. Thisproduces extra electric mobility toward downstream and increases thetransfer efficiencies in the very slow moving sheath flow often found inthis 2^(nd) DMA stage. Similarly, an electrode used in the 1^(st) DMAstage may be replaced by multiple electrodes to produce the electricfields in the same direction as the sheath flow to promote TargetParticle transfer efficiencies.

In another embodiment, the DMA apparatus has no inward cross flowthrough the gap, and particles are guided by filter electrodes whichalso eliminate unwanted particles which happen to pass through the gapby diffusion. By repetition of this process, this Q_(c) is kept minimalwhile achieving high transfer efficiency and resolving power. In oneembodiment, the Target Q_(c) is <0.7 ml/s. A cutaway of a cascade-typeDMA is depicted in FIG. 2. Here, the smaller particles are eliminated bythe guiding electrodes, which show a sharp cutoff in signals. Largerparticles with inward diffusion will be eliminated by the electrodesshown in left (high mass filter, Arrow 30). The guiding electrode willnot only function as the low mass filter, but also will “guide” theTarget Particles at the middle portion of tube to minimize wall loss atthe exit tube (Arrow 31). This is distinguishable from the related artbecause here, the non-target particles do not travel laterally from theouter cavity to the inner cavity. Here, the sheath flow is removedbefore the target particles are removed through the exit end. Ascompared to the related art, little migrating through the gaps (orslits) occurs. Due to the varying size of each annular and centralspace, the amount of air inserted through each space is adjusted so thatthe pressure exerted is substantially equal. In some related art, highvolume of air flow was pushed or pulled through a DMA, such as withsuction, along with the sample of target particles, from the outwardcavity to the inward cavity, draining at the exit end. Only a smallconcentration of the target particles was retrieved. Through the instantembodiment, a higher concentration of Target Particles are extracted.

In another embodiment, the DMA apparatus has outward cross flow, withthe amount of air inserted through the annular and central spaces beingsubstantially equal. Again, the non-target particles do not travellaterally from the outer cavity to the inner cavity, but rather from theinner cavity to the outer cavity. One significant advantage for thesetwo embodiments is that the exit flow is small, so that the flow rateexiting (with the target particles) is small. In one embodiment,approximately 1 mL/s exits, rather than 3 L/min from the related art.This results in highly concentrated target particles, which can then beused in a mass spectrometer or other analyzer.

FIG. 3 shows the separation of the groups of ions having the same m/z(mass/charge) ratio. Since MS (“Mass Spectrometry”) cannot separatethese groups, pre-filtering via an ionization method such aselectrospray (ESI) which produces highly charged molecules may becomeuseful. Note here that the ion counts in the simulation are reduced dueto the excessive Coulombic repulsion. It is due to the whole packetbeing released at the point source (the axisymmetric design of thisapparatus in this particular modeling process), but it does not happenin reality.

FIG. 3 shows one embodiment of the DMA 100 showing the housing 102.Inlet 96 provides aerosol particles 98 into the housing pathway 104 ofthe housing 102. Two conduits 106, 114 are located radially inwards fromthe housing 102. First conduit 106 forms a first pathway 118, such as anouter pathway. Second conduit 114 forms a second pathway 128, such as aninner pathway. Exit conduit 134 forms the exit pathway 138. The aerosolparticles 98 flow from inlet 96 longitudinally downstream to the exitpathway 138.

Outer walls 108, 112 separate the housing pathway 104 from the firstconduit pathway 118. The outer walls 108, 112 are located radiallyinward from the housing 102. Outer wall 108 is located longitudinallyupstream from the outer wall 112. A first outer aperture 110 is locatedbetween the outer walls 108, 112. The first outer aperture 110 providesaccess between the housing pathway 104 and the first pathway 118.

Inner walls 116, 122, 126 separate the first pathway 118 from the secondpathway 128. The inner walls 116, 122, 126 are located radially inwardfrom the housing 102 and the outer walls 108, 112. Inner wall 116 islocated longitudinally upstream from the inner walls 122, 126. Innerwall 122 is located longitudinally upstream from inner wall 126. A firstinner aperture 120 is located between the inner walls 116, 122. A secondinner aperture 124 is located between the inner walls 122, 126. Theinner apertures 120, 124 provide access between the first pathway 118and the second pathway 124.

Exit wall 136 separates the second pathway 128 from the exit pathway138. Block electrodes 130, 132 are located longitudinally upstream fromthe exit wall 136 and exit pathway 138.

Computational Methods.

A Statistical Jump Diffusion (1) (SJD) program and direct numericalsimulation package was used to transport the molecule by adding averagedrandom spread on the projectile's expected destination determined by thelocal electrostatic and aerodynamic field. This stochastic approach ispreferred over the direct deterministic trajectory approach, due to theastronomical number of collisions. Trajectories of large moleculesdepend on density, size, shape and other factors, and are characterizedby dimensionless Stoke, Knudsen and Péclet numbers. In this study, SJDhas been modified to satisfy the known particle behavior (i.e. aerosoldynamics) and tested in some well known velocity field calculated. Slipcorrection, relaxation time, and shape factor parameter are introduced.The diffusional spread is assumed to follow the square root law. Allelectrostatic fields are calculated by existing Laplace solver in SIMIONprogram. Finally, the gas flow inside the newly developed differentialmobility regions has been modeled by a commercial fluid dynamics packageto provide accurate particle trajectories.

Stoke friction, including Cunningham slip correction, is used as acorrection to predict the better approximation to frictional forcebetween fluid and a particle moving through this fluid. The slipcorrection factor can be defined as

$C = {1 + {\frac{2l}{d_{p}}( {1.257 + {0.4{\exp( \frac{{- 0.55}d_{p}}{l} )}}} )}}$

wherein the variables of the slip correction are:

C is the correction factor;

l is the mean free path; and

dp is the particle diameter.

The embodiment of the invention depicted in FIG. 4 increases the numberof electrode portions; for example, the lower segment of the medialelectrode includes an inner wall segment having electrode portions (1,3)through (1,8), and the middle section of the central electrode includeselectrode portions (2,2) through (2,6). The primary purpose of havingeach such section or segment (and/or portion thereof) that can receivean electrical voltage different and/or independent from that of anothersection or segment (and/or portion thereof) is to assist a forwardmotion of particles in relatively slow sheath gas, typically in the samedirection as the sheath gas flow. The success of detection or separationof Target Particles typically is a function of the differential reactionof different types of particles to the downstream flow of sheath gas(and/or sample carrier gas), each particle's size and electrical charge,and the relative intensities and lengths and configurations of theelectrical fields established within the airflow pathways. Suchdetection and/or separation of Target Particles may be maximized byoptimizing the voltage parameters of the system of such electrodesections and segments (and/or portions thereof), in accordance with thegas flow rate(s) and the size and charge of the Target Particles.

In one specific embodiment of the invention:

-   -   (a) said upstream segment of said medial electrode (1,1) has a        negative electrical voltage of about −105 volts; said downstream        segment of said medial electrode has an outer portion (1,2)        having a negative electrical voltage of about −105 volts; said        downstream segment of said medial electrode has successive        downstream inner portions having the following respective        negative voltages: (1,3)=−205 volts; (1,4)=−210 volts;        (1,5)=−215 volts; and (1,6)=−220 volts; (1,7)=−225 volts; and        (1,8)=−300 volts;    -   (b) said upper section of said central electrode (2,1) has a        negative electrical voltage of about −250 volts; said middle        section of said central electrode has successive downstream        portions having the following respective negative voltages:        (2,2)=−255 volts; (2,3)=−260 volts; (2,4)=−265 volts; (2,5)=−270        volts; and (2,6)=−275 volts; said lower section of said central        electrode (2,7) has a negative electrical voltage of about −285        volts; and    -   (c) said first block electrode (3,1) has a negative electrical        voltage of about −500 volts; said second block electrode has a        negative electrical voltage of about −495 volts; and said exit        electrode-tip (3,3) has a negative electrical voltage of about        −500 volts.

Those skilled in the art who have the benefit of this disclosure willappreciate that it may be used as the creative basis for designingdevices or methods similar to those disclosed herein, or to designimprovements to the invention disclosed herein; such new or improvedcreations should be recognized as dependent upon the invention disclosedherein, to the extent of such reliance upon this disclosure.

What is claimed is:
 1. A differential mobility analyzer apparatus foranalyzing a sample of aerosol particles or for separating andconcentrating aerosol particles, the apparatus comprising: a housingserving as a conduit for the flow of aerosol particles, the housingdefining a housing pathway for the flow of the particles along alongitudinal axis; an inlet providing the particles into the housing,the particles flowing longitudinally downstream; a first conduit locatedlaterally interior of the housing, the first conduit defining a firstpathway for the flow of the particles longitudinally downstream; asecond conduit located laterally interior of the first conduit, thesecond conduit defining a second pathway for the flow of the particleslongitudinally downstream; a first outer wall of the first conduit, thefirst outer wall separating the housing pathway from the first pathwaywherein a voltage is applied to the first outer wall; and a first innerwall of the second conduit, the first inner wall separating the firstpathway from the second pathway wherein a voltage is applied to thefirst inner wall; and a first block electrode that is electricallycharged wherein the block electrode at least partially obstructs thesecond pathway.
 2. The apparatus of claim 1 wherein the first outer wallterminates longitudinally upstream of the first inner wall.
 3. Theapparatus of claim 2 further comprising: a first opening in the firstconduit that starts at the termination of the first outer wall, thefirst opening in the first conduit providing access for the flow ofparticles between the housing pathway and the first pathway.
 4. Theapparatus of claim 3 further comprising: a first opening in the secondconduit that starts at a termination of the first inner wall, the firstopening in the second conduit providing access for the flow of particlesbetween the first pathway and the second pathway.
 5. The apparatus ofclaim 1 further comprising: a second outer wall of the first conduitlocated longitudinally downstream from the first outer wall, the secondouter wall separating the housing pathway from the first pathway whereina voltage is applied to the second outer wall.
 6. The apparatus of claim1 further comprising: a second inner wall of the second conduit locatedlongitudinally downstream from the first inner wall, the second innerwall separating the first pathway from the second pathway wherein avoltage is applied to the second inner wall.
 7. The apparatus of claim 6wherein the first outer wall terminates longitudinally upstream of thefirst inner wall.
 8. The apparatus of claim 6 further comprising: asecond opening in the second conduit that starts at the termination ofthe second inner wall, the second opening in the second conduitproviding access for the flow of particles between the first pathway andthe second pathway.
 9. The apparatus of claim 8 further comprising: anexit conduit located laterally interior of the second conduit, the exitconduit defining an exit pathway for the flow of the particleslongitudinally downstream, the exit conduit located laterally downstreamfrom the second opening in the second conduit.
 10. The apparatus ofclaim 9 further comprising: an exit wall of the exit conduit locatedlaterally downstream of the second opening in the second conduit whereina voltage is applied to the exit wall, the exit wall separating the exitpathway from the second pathway.
 11. The apparatus of claim 10 furthercomprising: a third inner wall of the second conduit locatedlongitudinally downstream from the second inner wall, the third innerwall separating the first pathway from the second pathway wherein avoltage is applied to the third inner wall, wherein the third inner wallstarts longitudinally upstream of the exit wall.
 12. The apparatus ofclaim 11 further comprising: the first block electrode located laterallyinward of the second conduit wherein the first block electrode starts ata position along the longitudinal axis equivalent to the second innerwall.
 13. The apparatus of claim 12 further comprising: a second blockelectrode located laterally inward of the second conduit wherein thesecond block electrode extends longitudinally downstream of the secondinner wall.
 14. The apparatus of claim 1 wherein the inlet introducesthe aerosol particles into the housing pathway.
 15. A differentialmobility analyzer apparatus for analyzing a sample of aerosol particlesor for separating and concentrating aerosol particles, the apparatuscomprising: a housing serving as a conduit for the flow of the aerosolparticles, the housing defining a housing pathway for the flow of theparticles longitudinally downstream, the housing having a cross sectionsized at a housing diameter; an inlet providing the particles into thehousing, the particles flowing longitudinally downstream; a first outerwall defining an outer conduit, the first outer wall located radiallyinterior of the housing, the first conduit defining an outer pathway forthe flow of the particles longitudinally downstream, the first conduithaving a cross section sized at a first diameter; the first outer wallseparating the outer pathway from the housing pathway wherein a voltageis applied to the first outer wall; a first inner wall defining an innerconduit located radially interior of the outer conduit, the innerconduit defining an inner pathway for the flow of the particleslongitudinally downstream, the inner conduit having a cross sectionsized at a second diameter; the first inner wall separating the innerpathway from the outer pathway wherein a voltage is applied to the firstinner wall; and a first block electrode that is electrically chargedwherein the block electrode is located longitudinally downstream fromthe first inner wall.
 16. The apparatus of claim 15 further comprising:wherein the first outer wall terminates longitudinally upstream of thefirst inner wall; a second outer wall forming a conduit having a crosssection with a diameter of the first diameter, the second outer walllocated longitudinally downstream from the first outer wall, the secondouter wall separating the housing pathway from the outer pathway whereina voltage is applied to the second outer wall; a second inner wallforming a conduit having a cross section with a diameter of the seconddiameter, the second inner wall located longitudinally downstream fromthe first inner wall, the second inner wall separating the outer pathwayfrom the inner pathway wherein a voltage is applied to the second innerwall; wherein the most upstream portion of the second outer wall startsat a position along the longitudinal axis equivalent to the mostupstream position of the second inner wall.
 17. The apparatus of claim16 further comprising: a first outer opening longitudinally between thefirst outer wall and the second outer wall wherein the first outeropening starts at the termination of the first outer wall, the firstouter opening providing access for the flow of particles between thehousing pathway and the outer pathway; a first inner openinglongitudinally between the first inner wall and the second inner wallwherein the first inner opening starts at the termination of the firstinner wall, the first inner opening providing access for the flow ofparticles between the outer pathway and the inner pathway.
 18. Theapparatus of claim 17 further comprising: a third inner wall forming aconduit having a having a cross section with a diameter of the seconddiameter, the third inner wall located longitudinally downstream fromthe first inner wall and the second inner wall, the third inner wallseparating the outer pathway from the inner pathway wherein a voltage isapplied to the third inner wall; a second inner opening longitudinallybetween the second inner wall and the third inner wall wherein thesecond inner opening starts at the termination of the second inner wall,the second inner opening providing access for the flow of particlesbetween the outer pathway and the inner pathway.
 19. The apparatus ofclaim 18 further comprising: an exit conduit located radially interiorof the third inner wall, the exit conduit defining an exit pathway forthe flow of the particles longitudinally downstream, the exit conduitlocated laterally downstream from the second inner opening, the exitconduit having a cross section sized at an exit diameter; an exit wallforming a conduit having a cross section sized at the exit diameter, theexit wall located laterally downstream of the second opening in thesecond conduit wherein a voltage is applied to the exit wall, the exitwall separating the exit pathway from the second pathway.
 20. Adifferential mobility analyzer apparatus for analyzing a sample ofaerosol particles or for separating and concentrating aerosol particles,the apparatus comprising: a housing serving as a conduit for the flow ofthe aerosol particles, the housing defining a housing pathway for theflow of the particles longitudinally downstream; an inlet providing theparticles into the housing, the particles flowing longitudinallydownstream; a first outer wall located laterally interior of thehousing, the first outer wall defining an outer pathway for the flow ofthe particles longitudinally downstream, the first outer wall separatingthe outer pathway from the housing pathway wherein a voltage is appliedto the first outer wall; a first inner wall located laterally interiorof the first outer wall, the first inner wall defining an inner pathwayfor the flow of the particles longitudinally downstream, the first innerwall separating the inner pathway from the outer pathway wherein avoltage is applied to the first inner wall; a second outer wall locatedlaterally interior of the housing, the second outer wall locateddownstream from the first outer wall, the second outer wall defining theouter pathway, the second outer wall separating the outer pathway fromthe housing pathway wherein a voltage is applied to the second outerwall; a second inner wall located laterally interior of the second outerwall, the second inner wall located downstream from the first innerwall, the second inner wall defining the inner pathway, the second innerwall separating the inner pathway from the outer pathway wherein avoltage is applied to the second inner wall; a third inner wall locatedlaterally interior of the second outer wall, the third inner walllocated downstream from the second inner wall, the third inner walldefining the inner pathway, the third inner wall separating the innerpathway from the outer pathway wherein a voltage is applied to the thirdinner wall; a first outer opening longitudinally between the first outerwall and the second outer wall, the first outer opening providing accessfor the flow of particles between the housing pathway and the outerpathway; a first inner opening longitudinally between the first innerwall and the second inner wall, the first inner opening providing accessfor the flow of particles between the outer pathway and the innerpathway; a second inner opening longitudinally between the second innerwall and the third inner wall, the second inner opening providing accessfor the flow of particles between the outer pathway and the innerpathway; and an exit conduit located laterally interior of the thirdinner wall, the exit conduit defining an exit pathway for the flow ofthe particles longitudinally downstream, the exit conduit locatedlaterally downstream from the second inner opening.